The major steps of the process of interaction between cytotoxic T lymphocytes (CTL) and target cells (TC) are Ca++-dependent. Presence of Ca++ is required for CTL-TC conjugate formation and increase in the intracellular free CA++ concentration is documented as an early feature of T-cell activation through T cell-receptor-T3 complex on the cell surface. It is not yet known, however, which proteins on the lymphocyte surface have Ca++-binding, Ca++-channel forming properties and Ca++-dependent functions. It is also not known which biochemical pathways are involved in T cell activation following an increase in concentration of intracellular Ca++. This project deals with these questions. We are attempting to isolate and to identify Ca++-binding and Ca++-dependent, calmodulin-regulated proteins in the cytoplasm and different domains of the plasma membranes of lymphocytes exploiting known physico-chemical properties of Ca++-binding proteins. Preliminary results indicate that mouse spleen lymphocytes appear to contain low levels of a calmodulin-dependent phosphatase, which is immunologically crossreactive and has similar molecular weight with a Ca++-dependent phosphatase from bovine brain, previously known as calcinueurin.